In rhizosphere

Leigh MB, JS Fletcher, X Fu, FJ Schmitz (2002) Root turnover an important source of microbial substrates in rhizosphere remediation of recalcitrant metabolites. Environ Sci Technol 36 1579-1583. 

Siciliano SD, JJ Germida (1999) Enhanced phytoremediation of chlorobenzoates in rhizosphere soil. Soil Biol Biochem 31 299-305. 

Root products, as defined by Uren and Reisenauer (17), represent a wide range of compounds. Only secretions are deemed to have a direct and immediate functional role in the rhizosphere. Carbon dioxide, although labeled an excretion, may play a role in rhizosphere processes such as hyphal elongation of vesicular-arbuscular mycorrhiza (39). Also, root-derived CO2 may have an effect on nonphotosynthetic fixation of CO2 by roots subject to P deficiency and thus contribute to exudation of large amounts of citrate and malate, as observed in white lupins (40). The amounts utilized are very small and, in any case, are extremely difficult to distinguish from endogenous CO2 derived from soil and rhizosphere respiration. 

D. L. Jones and P. R. Darrah, Influx and efflux of organic acids across the. soil-root interface of Zea mays L. and its implications in rhizosphere C flow. Plant Soil 173 103 (1995). 

During the last two decades, the functional characterization of plant root exudates involved in rhizosphere processes has attracted increased attention. The role of... 

G. Cieslinski, K. C. J. Van Rees, A. M. Szmigielska, G. S. R. Krishnamurti, and P. M. Huang. Low-molecular-weight organic acids in rhizosphere soils of durum wheat and their effect on cadmium bioaccumulation. Plant Soil 203 109 (1998). 

As already noted by Campbell and Greaves (16), the rhizosphere lacks physically precise delimitations and its boundary is hard to demarcate. Dimensions may vary with plant species and cullivar, stage of development, and type of soil. Soil moisture may affect the measurable size of the rhizosphere as well wetter soils may stick better to roots than drier soils (Fig, 1). This will change the volume of soil regarded as rhizosphere soil upon separation of rhizosphere from bulk soil and thus alter the measured concentration in rhizosphere and non-rhizosphere soil of a response variable in exudate concentration or microbial production. 

Figure 1 Scbematic representation of the dynamics rf a response variable, c.g., concentration of rbizodeposited C, in the rhizosphere (das/ied line) and ihe measured concentrations in rhizosphere and nonrhizosphere samples solid lines). The vertical airow indicates the separation of rhizosphere and nonrhizosphere soil the effect of soil moisture is indicated by horizontal arrows.

In general, one would expect that every enzyme will be released close to the roots either by roots or by microbes and eventually will be immobilized in rhizo.sphere soil. If a given enzyme activity has not yet been measured in rhizosphere soil, this is likely more due to the lack of specifically devoted studies or suitable methods than to a real absence. For example, a recent study has first developed an assay for myrosinase in soil and, second, has hypothesized the... 

Enhanced nutrient cycling in both the rhizosphere and bulk soil may depend on the bacterial grazing by protozoa or nematodes with release of inorganic N. Nematodes appear to be the primary consumers of bacteria in the rhizosphere, whereas protozoa are equally prevalent in rhizosphere and bulk soil (41,97). Estimated C-to-N ratios of bacterial-feeding nematodes range from 5 1 to 10 1 (98,99) and are generally higher than those of their bacterial food source thus the excess N is excreted as ammonia (100,101) by nematodes. The estimated... 

J. E. Loper and M. D. Henkels, Availability of iron to Pseudomonas fliiorescens in rhizosphere and bulk soil evaluated with an ice nucleation reporter gene. Appl. Environ. Microbiol. 63 99 (1997). 

The importance of including soil-based parameters in rhizosphere simulations has been emphasized (56). Scott et al. u.sed a time-dependent exudation boundary condition and a layer model to predict how introduced bacteria would colonize the root environment from a seed-based inoculum. They explicitly included pore size distribution and matric potential as determinants of microbial growth rate and diffusion potential. Their simulations showed that the total number of bacteria in the rhizosphere and their vertical colonization were sensitive to the matric potential of the soil. Soil structure and pore size distribution was also predicted to be a key determinant of the competitive success of a genetically modified microorganism introduced into soil (57). The Scott (56) model also demonstrated that the diffusive movement of root exudates was an important factor in determining microbial abundance. Results from models that ignore the spatial nature of the rhizosphere and treat exudate concentration as a spatially averaged parameter (14) should therefore be treated with some caution. 

G. D. Bowen, and A. D. Rovira, Are modelling approaches useful in rhizosphere ecology Modern Methods in the Study of Microbial Ecology (T. Rosswall, ed.). Bull. Ecol. Res. Com. (Stockholm) /7 443 (1973). 

J. Swinnen, J. A. van Veen, and R. Merckx. C pulse-labelling of field-grown spring wheat an evaluation of its use in rhizosphere carbon budget estimations. Soil Biol. Biochem. 25 161 (1994). 

Shuman L.M., Wang J. Effect of rice variety on zinc, cadmium, iron and manganese content in rhizosphere and non-rhizosphere soils fractions. Commun. Soil Sci Plant Anal 1997 28 23-36. 

Zhang TC, Pang H. Applications of microelectrode techniques to measure pH and oxidation-reduction potential in rhizosphere. Soil Environ. Sci. Technol. 1999 33 1293-1299. 

E. coli 0157 H7 in soil. In unplanted, fallow soils, . coli 0157 H7 persisted only for 25-41 days, but was found up to 92 and 96 days if alfalfa or rye plants were present, respectively. Ibeweke et al. (2004) also found that the presence of roots in contaminated soils increased concentrations of E. coli 0157 H7. In this study, E. coli introduced through irrigation water was found to reach higher densities in rhizosphere soils than in nonrhizo-sphere soils (Ibeweke et al, 2004). Bacterial populations in soil increased after the addition of plant material to soil the bacterial population spiked and then fluctuated in a wave-like fashion that was not found to be the result of nitrogen shortages or pH (Ibeweke et al, 2004). In contrast to... 

Whatever the cause of the changes in rhizosphere pH, the corresponding increase or decrease of proton concentration will promote the dissolution or precipitation of a range of soil minerals the direct implication of root-induced release of protons in the dissolution of phosphates, silicates, or oxides has been reported (Hinsinger et al., 1993 Hinsinger and Gilkes, 1996 Bertrand and Hinsinger, 2000). 

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